Using Digilent’s PMOD8LD with ZYBO FPGA board

This article explain how capabilities of powerful Xilinx FPGA ZYBO board can be expanded by using Pmod extensions from Digilent.

Xilinx FPGA ZYBO board is perfect tool to learn FPGA programming. It is powerful but also a little bit limited if you compare to others boards from competitors. One of its limitation is amount of LED. Only 4 green LED available on board, luckily we have 6 PMOD extension ports!

Pmod connectors

As ZYBO reference manual tells: Pmod connectors are 2×6, right-angle, 100-mil spaced female connectors that mate with standard 2×6 pin headers. Each 12-pin Pmod connector provides two 3.3V VCC signals (pins 6 and 12), two Ground signals (pins 5 and 11), and eight logic signals, as shown in Fig. 16. The VCC and Ground pins can deliver up to 1A of current, but care must be taken not to exceed any of the power budgets of the onboard regulators or the external power supply.

1. JA Dual Analog/Digital Pmod (XADC Pmod)

The on-board Pmod expansion connector labeled “JA” is wired to the auxiliary analog input pins of the PL. Depending on the configuration, this connector can be used to input differential analog signals to the analog-to-digital converter inside the Zynq (XADC). Any or all pairs in the connector can be configured either as analog input or digital input-output. For more information on using the XADC core, refer to the Xilinx document titled “7 Series FPGAs and Zynq-7000 All Programmable SoC XADC Dual 12-Bit 1 MSPS Analog-to-Digital Converter.” It is also possible to access the XADC core directly using the PS, via the “PS-XADC” interface. This interface is described in full in chapter 30 of the Zynq Technical Reference Manual.

2(JB), 3(JC), 4(JD) – High-Speed Pmods

High-speed Pmods use standard Pmod connector but have no protection(with loading resistors) against short circuits but allow much faster speeds. The signals are apired to the adjacent signals in same row: pins 1 and 2, 3 and 4, 7 and 8, 9 and 10.
Use it only if Standard Pmod is occupied.

5. JE Standard Pmod

The standard Pmod connector is connected to the PL of the Zynq via 200 Ohm series resistors. The series resistors prevent short circuits that can occur if the user accidently drives a signal that is supposed to be used as an input. The downside to this added protection is that these resistors can limit the maximum switching speed of the data signals. If the Pmod being used does not require high-speed access, then the standard Pmod connector should be used to help prevent damage to the devices.

6. JF MIO Pmod

MIO Pmod connected to MIO bus in PS of Zynq via 200Ohm resistors. Since signals connected to MIO interface only GPIO, UART, I2C and SPI cores can be used to access devices connected to this Pmod.

Pmod pinout

It is important to create correct entries in XDC constraints file according to table from Reference Manual. Example of XDC for Standard Pmod port at the end of article.